Diamondback scopes are pretty much entry level scopes, I prefer the Vipers for my needs (a balance between quality, options, and cost) but the Diamondback glass and construction is acceptable for entry level use. The truth is that most shooters don't need the highest quality scopes, they wouldn't even be able to tell the difference between the performance of a high end scope vs. an entry level scope. If you plan on shooting at medium sized game animals at typical distances then you can pretty much use any scope that is in the $300 to $800 range and be very happy with it.
BDC scopes aren't exact science, well they are but it may not be the exact science that works with your rifle and cartridge. BDC scopes are based on the trajectory of a specific chambering, bullet weight, velocity, and ambient conditions, if any of those things don't match with your setup then the BDC marks wont be accurate. I use a Nikon BDC scope but I also use their software to develop accurate range information when I use different bullets.
Finding a quality scope isn't hard, you get what you pay for but some things are obvious when you buy the really cheap scopes ($130);
The image wont be sharp and clear over the entire view (especially the edges of your view).
The knobs wont feel crisp and sharp when you adjust them.
The lens coating wont be consistent across the entire surface.
The scope feels light and unreliable for daily use in the field.
The biggest mistake that most of us make when choosing a scope is not looking through the scope under low light conditions outside, we typically pick up the scope at the store and look through it inside the building under artificial lighting - which does not prove much of anything about the scope's ability to perform out of doors. You need to take the scope outside and look at distant details, preferably at dawn or dusk.
Check
this link out for a good instruction on picking a good scope.